In 1981, Arthur Fleck, a 40-year-old clown lived with his mother, Penny in the chaotic city of Gotham. She has a brain disorder that causes her to laugh at the wrong time and she often visits social service workers to get her medication. After a group of street children steal Arthur’s boards and gang up on him in the hallway, one of his coworkers lends a pistol as a means of self-protection. One time, his illness recurred while he was riding the train, so Sophie was amazed by his behavior and Arthur gave a card to answer his surprise. Arthur then established a good relationship with him, which he later realized that Sophie lived in the same apartment with him.
During a visit to the children’s hospital, Arthur’s gun fell from his pocket, so he was fired from his job. Arthur also learned that the social service program was closed, so he had no way to get medicine. On his way home by subway, Arthur was beset by three young Wall Street businessmen, so he shot them dead with the pistol he had borrowed. Arthur did not realize that the assassination would begin a demonstration against the rich in the city by wearing a clown mask. A few days earlier, Thomas Wayne had nominated himself as mayor because he felt uneasy with the chaos in the city that never stopped.
Sophie attended Arthur’s lone joke show which went very badly. Arthur laughed uncontrollably and had difficulty conveying his joke. A popular talk show host, Murray Franklin, aired the video live as a mockery. Arthur stole a letter his mother wrote to Thomas Wayne who was one of the most respected people in the city and found himself to be Wayne’s illegitimate son. He cursed his mother for keeping this a secret from him and shortly after, Penny fell ill and was hospitalized. Arthur was also harassed by two detectives who were suspicious of his involvement in the subway shooting, but he denied it.
The situation in the city became even more chaotic with protesters pouring in everywhere. When the protesters began fighting with security officers, Arthur sneaked into a building where a special event for famous personalities was held. Arthur met with Thomas Wayne and questioned his status with Thomas. Thomas said that Penny was crazy and not even Arthur’s biological mother, slapping Arthur afterwards. Arthur visits Arkham Hospital to find out the truth of Thomas’s words. Arthur steals Penny’s case file and discovers that he was indeed adopted after being abandoned as a baby. He also learned that Penny was abusive to him when he was a child, including serious head trauma which resulted in pathological laughter. Arthur returned to the hospital and immediately covered his mother’s head with a pillow until he ran out of air. He returned to his apartment building and entered Sophie’s room. Sophie was surprised by his presence and asked him to leave. Arthur then realized that his experience with Sophie was merely an illusion.
An employee from the Murray Franklin program called and asked Arthur to appear on the show. Arthur agreed and planned to commit suicide on the show. When he prays and wears his clothes, he who holds the small scissors is visited by two old colleagues who want to offer condolences for the death of his mother. Arthur even pierced the neck and eyes of one of them with the scissors, then banged his head many times to death right then and there. Arthur let the other one live because of his kindness in the past. On the way to the studio, he was chased by two detectives to a train full of clown protesters. One of the detectives accidentally shot dead a protester, so the other protesters began to gang up on the two detectives to critical, and Arthur escaped from the commotion.
Before the event began, Arthur asked Murray to introduce him as the Joker, a mock Murray some time ago towards him. The program proceeded smoothly as usual, but Arthur continued to acknowledge the assassination on the subway and questioned the hypocrisy of the people who flattered the three young men while demeaning him, citing the public to prefer stepping on his body on the highway instead of burying it properly. Murray tried to calm the atmosphere, but Arthur ignored him. Arthur immediately shot Murray dead in the head right then and there, so many spectators ran away scared and he was arrested by the police for it. On the way to the police station, Arthur sees Gotham being beset by chaos by protesters. One of the pointers